Refrigeration apparatus

ABSTRACT

A refrigerant circuit ( 10 ) of a refrigeration apparatus is filled up with carbon dioxide as a refrigerant. In the refrigerant circuit ( 10 ), a first compressor ( 21 ) and a second compressor ( 22 ) are arranged in parallel. The first compressor ( 21 ) is connected to both an expander ( 23 ) and a first electric motor ( 31 ), and is driven by both of the expander ( 23 ) and the first electric motor ( 31 ). On the other hand, the second compressor ( 22 ) is connected only to a second electric motor ( 32 ), and is driven by the second electric motor ( 32 ). In addition, the refrigerant circuit ( 10 ) is provided with a bypass line ( 40 ) which bypasses the expander ( 23 ). The bypass line ( 40 ) is provided with a bypass valve ( 41 ). And, the capacity of the second compressor ( 22 ) and the valve opening of the bypass valve ( 41 ) are regulated so that the COP of the refrigeration apparatus is improved after enabling the refrigeration apparatus to operate properly in any operation conditions.

TECHNICAL FIELD

The present invention generally relates to refrigeration apparatuseswhich perform refrigeration cycles and more specifically to arefrigeration apparatus which is provided with an expander for producingpower by the expansion of refrigerant.

BACKGROUND ART

There is a conventionally known refrigeration apparatus of the typewhich performs a refrigeration cycle by circulating refrigerant througha refrigerant circuit which is a closed circuit. Such a type ofrefrigeration apparatus has been used widely as an air conditioner orother like apparatus. For example, Japanese Patent Application KokaiPublication No. 2001-107881 discloses one such refrigeration apparatusin which the high pressure of a refrigeration cycle is set higher thanthe critical pressure of a refrigerant. This refrigeration apparatusincludes, as a mechanism for expanding refrigerant, an expander formedby fluid machinery of the scrolled type. And, the expander is connectedto a compressor by a shaft, with a view to accomplishing improvement inCOP (coefficient of performance) by making utilization of power producedin the expander for driving the compressor.

In the refrigeration apparatus disclosed in the aforesaid gazette, themass flow rate of refrigerant that passes through the expander becomesconstantly equal to the mass flow rate of refrigerant that passesthrough the compressor. This is because the refrigerant circuit isformed by a closed circuit. On the other hand, both the density ofrefrigerant at the entrance of the expander and the density ofrefrigerant at the entrance of the compressor vary, depending on theoperation condition of the refrigeration apparatus. In the refrigerationapparatus of the aforesaid gazette, however, the expander and thecompressor are connected together, and it is impossible to make theratio between the displacement volume of the expander and thedisplacement volume of the compressor variable. This gives rise to aproblem that, when there are changes in operating condition, it becomesimpossible for the refrigeration apparatus to continue to operatestably.

To cope with this problem, Japanese Patent Application Kokai PublicationNo. 2001-116371 proposes a technique of providing in the refrigerantcircuit a bypass line that bypasses an expander. Stated another way, ifthe displacement volume of the expander is insufficient, a portion ofrefrigerant that has dissipated heat is made to flow into the bypassline for assuring the circulation amount of refrigerant, with a view toenabling a refrigeration cycle to continue in stable manner.

But in reality the displacement volume of the expander may becomeexcessive depending on the operation condition of the refrigerationapparatus. Also in this case, it becomes impossible for therefrigeration apparatus to continue to operate stably. A measure forthis problem is disclosed by Fukuda, Mitsuhiro and two others in a paperentitled “THEORETICAL PERFORMANCE OF CARBON DIOXIDE CYCLE WITHINCORPORATION OF COMPRESSOR/EXPANDER INTEGRATED TYPE FLUID MACHINERY”,35^(th) Air Conditioning and Refrigeration Combined Lecture Meeting,Lecture Collected Papers, pp. 57-60. More specifically, in thisnon-patent document, in order to deal with the problem, an expansionvalve is disposed upstream of an expander in addition to a bypass linethat bypasses the expander. To sum up, refrigerant traveling in thedirection of the expander is decompressed by the expansion valve. Thatis, the specific volume of refrigerant flowing into the expander isincreased beforehand, with a view to enabling a refrigeration cycle tocontinue in stable manner.

PROBLEMS THAT INVENTION INTENDS TO SOLVE

If, as is proposed in the aforesaid non-patent document, a refrigerantcircuit is provided with a bypass line that bypasses an expander, and anexpansion valve that is positioned upstream of the expander, thisarrangement makes it possible to perform refrigeration cycles in anyoperation conditions. However, the problem is that the production ofpower in the expander is reduced, thereby degrading the COP (coefficientof performance) of the refrigeration apparatus.

Here, with reference to FIG. 6, the above-described problem isdiscussed. FIG. 6 shows a relationship between the refrigerantevaporation temperature and the COP on condition that the temperatureand the pressure of high-pressure refrigerant are constant at the exitof a radiator. Suppose every portion of refrigerant exiting the radiatorflows into the expander as it is. In this case, the production of powerin the expander increases to the full and the COP of the refrigerationapparatus increases to the greatest possible level. FIG. 6 shows arelationship between the refrigerator apparatus COP and the refrigerantevaporation temperature in such a supposed ideal state, as indicated bythe chain double-dashed line.

Let's say, the displacement volume of the expander and that of thecompressor are set based on an operation condition (refrigerantevaporation temperature=0° C.). At this time, in an operation conditionin which refrigerant evaporates at a temperature of 0° C., every portionof refrigerant exiting the radiator flows into the expander as it is,and the COP of the refrigeration apparatus increases to the greatestpossible level.

However, if the evaporation temperature of refrigerant exceeds 0° C.,this causes the low pressure of the refrigeration cycle to increase.Consequently, the density of refrigerant at the entrance of thecompressor increases. This results in a state wherein the displacementvolume of the expander becomes too small relative to that of thecompressor, and a portion of refrigerant exiting the radiator has to beflowed into the bypass line. Therefore, the production of power in theexpander is reduced and, as indicated by the solid line of FIG. 6, theCOP of the refrigeration apparatus degrades when compared to the idealstate's value.

On the other hand, if the evaporation temperature of refrigerant fallsbelow 0° C., this causes the low pressure of the refrigeration cycle todecrease. Consequently, the density of refrigerant at the entrance ofthe compressor decreases. This results in a state wherein thedisplacement volume of the expander becomes too great relative to thatof the compressor, and refrigerant exiting the radiator has to be flowedinto the expander after pre-expansion by the expansion valve. Therefore,also in this case, the production of power in the expander is reducedand, as indicated by the solid line of FIG. 6, the COP of therefrigeration apparatus degrades when compared to the ideal state'svalue.

Bearing in mind these problems with the prior art techniques, thepresent invention was made. Accordingly, an object of the presentinvention is to improve the COP of a refrigeration apparatus afterenabling the refrigeration apparatus to operate properly in anyoperation conditions.

DISCLOSURE OF INVENTION

A first invention is directed to a refrigeration apparatus whichperforms a refrigeration cycle by circulating refrigerant through arefrigerant circuit (10). The refrigeration apparatus of the firstinvention comprises: an expander (23), disposed in the refrigerantcircuit (10), for producing power by expansion of high-pressurerefrigerant; a first compressor (21), disposed in the refrigerantcircuit (10) and connected to a first electric motor (31) and theexpander (23), for compressing refrigerant when driven by power producedin the first electric motor (31) and the expander (23); and, a variablecapacity second compressor (22), disposed in parallel with the firstcompressor (21) in the refrigerant circuit (10) and connected to asecond electric motor (32), for compressing refrigerant when driven bypower produced in the second electric motor (32).

A second invention provides a refrigeration apparatus according to therefrigeration apparatus of the first invention. The refrigerationapparatus of the second invention is characterized in that it furthercomprises a control means (50) for regulating the capacity of the secondcompressor (22) so that the high pressure of the refrigeration cycleassumes a predetermined target value.

A third invention provides a refrigeration apparatus according to therefrigeration apparatus of the first invention. The refrigerationapparatus of the third invention is characterized in that it furthercomprises a bypass passage (40) for establishing fluid communicationbetween an entrance and exit sides of the expander (23) in therefrigerant circuit (10); and a control valve (41) for regulating theflow rate of refrigerant in the bypass passage (40).

A fourth invention provides a refrigeration apparatus according to therefrigeration apparatus of the third invention. The refrigerationapparatus of the fourth invention is characterized in that it furthercomprises a control means (50) for regulating the capacity of the secondcompressor (22) and the valve opening of the control valve (41) so thatthe high pressure of the refrigeration cycle assumes a predeterminedtarget value.

A fifth invention provides a refrigeration apparatus according to therefrigeration apparatus of the fourth invention. The refrigerationapparatus of the fifth invention is configured so that: when the controlvalve (41) is in the fully closed state and the high pressure of therefrigeration cycle falls below the predetermined target value, thecontrol means (50) sets the second compressor (22) in operation andregulates the capacity of the second compressor (22) while, on the otherhand, when the second compressor (22) is in the stopped state and thehigh pressure of the refrigeration cycle exceeds the predeterminedtarget value, the control means (50) places the control valve (41) inthe open state and regulates the valve opening of the control valve(41).

A sixth invention provides a refrigeration apparatus according to therefrigeration apparatus of any one of the first to fifth inventions. Therefrigeration apparatus of the sixth invention is characterized in thatthe refrigerant circuit (10) is filled up with carbon dioxide as arefrigerant, and that the high pressure of the refrigeration cycleperformed by circulating refrigerant through the refrigerant circuit(10) is set higher than the critical pressure of carbon dioxide.

Operation

In the first invention, refrigerant circulates through the refrigerantcircuit (10), wherein the refrigerant repeatedly undergoes a sequence ofprocesses (that is, compression, dissipation of heat, expansion, andabsorption of heat), and a refrigeration cycle is performed. The processof expanding refrigerant is carried out in the expander (23). Morespecifically, in the expander (23), high-pressure refrigerant after heatdissipation expands, and power is recovered from the high-pressurerefrigerant. The process of compressing refrigerant is carried out bythe first compressor (21) or the second compressor (22). When both thefirst compressor (21) and the second compressor (22) are operated, oneportion of refrigerant after heat absorption is drawn into the firstcompressor (21) while on the other hand, the remaining portion is drawninto the second compressor (22). The first compressor (21) is driven bypower recovered in the expander (23) and power generated by the firstelectric motor (31), and compresses the refrigerant drawn thereinto. Onthe other hand, the second compressor (22) is driven by power generatedby the second electric motor (32), and compresses the refrigerant drawnthereinto.

In the first invention, the first compressor (21) is connected to theexpander (23). Therefore, the first compressor (21) is constantly inoperation when the refrigeration apparatus is in operation. On the otherhand, the second compressor (22), which is not connected to the expander(23), is driven by the second electric motor (32), and is variable inits capacity. During the operation of the refrigeration apparatus, thecapacity of the second compressor (22) is regulated according to need.In other words, the second compressor (22) may possibly be at restduring the operation of the refrigeration apparatus.

In the second invention, the control means (50) regulates the capacityof the second compressor (22). Regulation of the capacity of the secondcompressor (22) by the control means (50) is made in order to bring thehigh pressure of the refrigeration cycle to a predetermined targetvalue. For example, if the high pressure of the refrigeration cycle ishigher than the target value, the control means (50) performs anoperation of reducing the capacity of the second compressor (22). On theother hand, if the high pressure of the refrigeration cycle is lowerthan the target value, the control means (50) performs an operation ofincreasing the capacity of the second compressor (22).

In the third invention, the refrigerant circuit (10) is provided withthe bypass passage (40) and the control valve (41). When the controlvalve (41) is in the open state, one portion of high-pressurerefrigerant after heat dissipation flows into the bypass passage (40),and the remainder flows into the expander (23). As the valve opening ofthe control valve (41) is varied, the inflow amount of refrigerant intothe bypass passage (40) varies.

In the fourth invention, the control means (50) regulates the capacityof the second compressor (22) and the valve opening of the control valve(41). The controlling of the capacity of the second compressor (22) andthe controlling of the valve opening of the control valve (41) by thecontrol means (50) are performed in order for the high pressure of therefrigeration cycle to assume a predetermined target value. For example,if the high pressure of the refrigeration cycle is greater than thetarget value, the control means (50) performs an operation of decreasingthe capacity of the second compressor (22) or an operation of increasingthe valve opening of the control valve (41) while, on the other hand, ifthe high pressure of the refrigeration cycle is smaller than the targetvalue, the control means (50) performs an operation of increasing thecapacity of the second compressor (22) or an operation of decreasing thevalve opening of the control valve (41).

In the fifth invention, the control means (50) performs the followingoperation. That is, the control means (50), only when any one of thesecond compressor (22) and the control valve (41) becomesuncontrollable, performs control operations on the other.

More specifically, when the high pressure of the refrigeration cyclefalls below the target value, with the control valve (41) opened, thecontrol means (50) gradually reduces the valve opening of the controlvalve (41). And, if the high pressure of the refrigeration cycle isstill lower than the target value even when the control valve (41) isfully closed, then the control means (50) activates the secondcompressor (22) and starts regulating the capacity of the secondcompressor (22).

On the other hand, when the high pressure of the refrigeration cycle ishigher than the target value, with the second compressor (22) operated,the control means (50) gradually reduces the capacity of the secondcompressor (22). And, if the high pressure of the refrigeration cycle isstill higher than the target value even when the second compressor (22)is brought to a stop, then the control means (50) places the controlvalve (41) in the open state and starts regulating the valve opening ofthe control valve (41).

Thus, in the fifth invention, the second compressor (22) is operatedonly when the control valve (41) is in the fully closed state, and thecontrol valve (41) is opened only when the second compressor (22) is atrest.

In the sixth invention, the refrigerant circuit (10) uses carbon dioxide(CO₂) as a refrigerant. This carbon dioxide refrigerant is compressed inthe first compressor (21) or in the second compressor (22) to a pressurelevel higher than its critical pressure. Carbon dioxide of higherpressure than its critical pressure flows into the expander (23).

Working Effect

In the refrigerant circuit (10) of the refrigeration apparatus of thepresent invention, the second compressor (22) which is not connected tothe expander (23) is arranged in parallel with the first compressor(21). Therefore, even in such an operation condition that the volume ofdisplacement only by the first compressor (21) connected to the expander(23) becomes deficient, it is possible to compensate such a deficiencyby setting the second compressor (22) in operation, and therefrigeration cycle is continued in an adequate operation condition.And, even in an operation condition in which refrigerant has to beflowed into the expander (23) after being pre-expanded by an expansionvalve or the like as conventionally required, it is possible tointroduce high-pressure refrigerant after heat dissipation into theexpander (23) without the necessity for pre-expansion. As a result, thedegradation of power produced in the expander (23) is avoided.

That is, in accordance with the present invention, even in an operationcondition in which there is, conventionally, no other choice but tosacrifice the COP of the refrigeration apparatus in order to assurecontinuation of the refrigeration cycle in an adequate operationcondition, it becomes possible to hold the COP of the refrigerationapparatus at high levels while, simultaneously, assuring continuation ofthe refrigeration cycle. Therefore, in accordance with the presentinvention, the refrigeration apparatus operates in stable manner,regardless of the operation condition, whereby the COP of therefrigeration apparatus is improved.

In accordance with the third invention, the refrigerant circuit (10) isprovided with the bypass passage (40) and the control valve (41). Here,for the case of compressors variable in capacity, generally there existrestrictions on the capacity variable range. This may give rise to anoperation condition in which it is impossible to enable therefrigeration cycle to continue in an adequate condition by onlyregulation of the capacity of the second compressor (22), depending onthe status of use of the refrigeration apparatus. On the other hand, inaccordance with the present invention, it becomes possible to achievestable continuation of the refrigeration cycle even in such an operationcondition by regulating the rate of inflow of high-pressure refrigerantinto the bypass passage (40). To sum up, even in an operation conditionin which the displacement volume of the expander (23) alone is notsufficient enough to secure a required circulation amount ofrefrigerant, a deficiency in the refrigerant mass flow rate is coveredby introduction of high-pressure refrigerant into the bypass passage(40), thereby making it possible to assure continuation of therefrigeration cycle in an adequate operation condition.

In accordance with the fifth invention, it is arranged that, only whenthe second compressor (22) is stopped and its capacity regulationbecomes impossible to make, the control valve (41) is opened forintroduction of high-pressure refrigerant into the bypass passage (40).As a result of such arrangement, it becomes possible to minimize thefrequency of falling into an operation state in which power produced inthe expander (23) is lowered because the amount of inflow of refrigerantis reduced, thereby enabling the refrigeration apparatus to operate inan operation state capable of making the COP of the refrigerationapparatus as high as possible.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a piping system diagram showing an arrangement of arefrigerant circuit in a first embodiment;

FIG. 2 is a Mollier chart (pressure-enthalpy diagram) showing arefrigeration cycle in the refrigerant circuit of the first embodiment;

FIG. 3A is a Mollier chart (pressure-enthalpy diagram) showing arefrigeration cycle in the refrigerant circuit of the first embodimentduring the space cooling mode of operation when the temperature ofoutside air decreases;

FIG. 3B is a Mollier chart (pressure-enthalpy diagram) showing arefrigeration cycle in the refrigerant circuit of the first embodimentduring the space heating mode of operation when the temperature ofoutside air decreases;

FIG. 4A is a Mollier chart (pressure-enthalpy diagram) showing arefrigeration cycle in the refrigerant circuit of the first embodimentduring the space cooling mode of operation when the temperature ofoutside air increases;

FIG. 4B is a Mollier chart (pressure-enthalpy diagram) showing arefrigeration cycle in the refrigerant circuit of the first embodimentduring the space heating mode of operation when the temperature ofoutside air increases;

FIG. 5 is a piping system diagram showing an arrangement of arefrigerant circuit in a second embodiment; and

FIG. 6 shows a relationship between the refrigerant evaporationtemperature and the coefficient of performance (COP) in a conventionalrefrigeration apparatus.

BEST MODE FOR CARRYING OUT INVENTION

Hereafter, embodiments of the present invention will be described indetail with reference to the drawing figures.

Embodiment 1 of Invention

Referring to FIG. 1, a first embodiment is an air conditioner that isformed by a refrigeration apparatus according to the present invention.The air conditioner of the first embodiment includes a refrigerantcircuit (10) and a controller (50) which is a control means. And, theair conditioner of the present embodiment is so configured as to causerefrigerant to circulate through the refrigerant circuit (10), therebyto switchably provide space cooling or space heating.

The refrigerant circuit (10) is filled up with carbon dioxide (CO₂) as arefrigerant. Moreover, the refrigerant circuit (10) is provided with anindoor heat exchanger (11), an outdoor heat exchanger (12), a firstfour-way switching valve (13), a second four-way switching valve (14), afirst compressor (21), a second compressor (22), and an expander (23).

The indoor heat exchanger (11) is formed by a fin and tube heatexchanger of the so-called cross fin type. The indoor heat exchanger(11) is supplied with indoor air by a fan (not shown in the figure). Inthe indoor heat exchanger (11), heat exchange takes place between indoorair supplied by the fan and refrigerant in the refrigerant circuit (10).In the refrigerant circuit (10), one end of the indoor heat exchanger(11) is connected, by piping, to a first port of the first four-wayswitching valve (13) and the other end is connected, by piping, to afirst port of the second four-way switching valve (14).

The outdoor heat exchanger (12) is formed by a fin and tube heatexchanger of the so-called cross fin type. The outdoor heat exchanger(12) is supplied with outdoor air by a fan (not shown in the figure). Inthe outdoor heat exchanger (12), heat exchange takes place betweenoutdoor air supplied by the fan and refrigerant in the refrigerantcircuit (10). In the refrigerant circuit (10), one end of the outdoorheat exchanger (12) is connected, by piping, to a second port of thefirst four-way switching valve (13) and the other end is connected, bypiping, to a second port of the second four-way switching valve (14).

Both the first compressor (21) and the second compressor (22) are formedby fluid machines of the rolling piston type. In other words, these twocompressors (21, 22) are formed by fluid machines of the displacementtype whose displacement volume is constant. In the refrigerant circuit(10), discharge sides of the first and second compressors (21, 22) areconnected, by piping, to a third port of the first four-way switchingvalve (13) and their suction sides are connected, by piping, to a fourthport of the first four-way switching valve (13). Thus, in therefrigerant circuit (10), the first compressor (21) and the secondcompressor (22) are connected in parallel with each other.

The expander (23) is formed by a fluid machine of the rolling pistontype. That is, the expander (23) is formed by a fluid machine of thedisplacement type whose displacement volume is constant. In therefrigerant circuit (10), an inflow side of the expander (23) isconnected, by piping, to a third port of the second four-way switchingvalve (14) and its outflow side is connected, by piping, to a fourthport of the second four-way switching valve (14).

The compressors (21, 22) and the expander (23) are not limited to fluidmachinery of the rolling piston type. In other words, for example,displacement fluid machines of the scroll type may be used to constitutethe compressors (21, 22) and the expander (23).

The first compressor (21) is connected, through a drive shaft, to theexpander (23) and a first electric motor (31). The first compressor (21)is rotationally driven by both power produced by expansion ofrefrigerant in the expander (23) and power generated by energization tothe first electric motor (31). In addition, since the first compressor(21) and the expander (23) which are connected together by the singledrive shaft, they rotate at the same speed. Stated another way, theratio between the displacement volume of the first compressor (21) andthe displacement volume of the expander (23) is constant at all times.

On the other hand, the second compressor (22) is connected, through adrive shaft, to a second electric motor (32). This second compressor(22) is rotationally driven only by power generated by energization tothe second electric motor (32). That is, the second compressor (22) isallowed to operate at a different revolving speed from that of the firstcompressor (21) and the expander (23).

The first electric motor (31) and the second electric motor (32) areeach supplied with alternating-current (AC) power having a predeterminedfrequency from a respective inverter (not shown). The frequency of ACpower that is supplied to the first electric motor (31) and thefrequency of AC power that is supplied to the second electric motor (32)are set individually.

If the frequency of AC power that is supplied to the first electricmotor (31) is changed, this causes the revolving speed of the firstcompressor (21) and the expander (23) to vary and, as a result, thefirst compressor (21) and the expander (23) each undergo a variation intheir displacement volume. That is, the first compressor (21) and theexpander (23) are variable in capacity. On the other hand, if thefrequency of AC power that is supplied to the second electric motor (32)is changed, this causes the revolving speed of the second compressor(22) to vary and, as a result, the second compressor (22) undergoes achange in displacement volume. That is, the second compressor (22) isvariable in capacity.

As described above, the first to fourth ports of the first four-wayswitching valve (13) are, respectively, connected to the indoor heatexchanger (11), to the outdoor heat exchanger (12), to the dischargesides of the first and second compressors (21, 22), and to the suctionsides of the first and second compressors (21, 22). The first four-wayswitching valve (13) is switchable between a first state that permitsfluid communication between the first port and the fourth port and fluidcommunication between the second port and the third port (as indicatedby the solid line of FIG. 1), and a second state that permits fluidcommunication between the first port and the third port and fluidcommunication between the second port and the fourth port (as indicatedby the broken line of FIG. 1).

On the other hand, the first to fourth ports of the second four-wayswitching valve (14) are, respectively, connected to the indoor heatexchanger (11), to the outdoor heat exchanger (12), to the inflow sideof the expander (23), and to the outflow side of the expander (23). Thesecond four-way switching valve (14) is switchable between a first statethat permits fluid communication between the first port and the fourthport and fluid communication between the second port and the third port(as indicated by the solid line of FIG. 1), and a second state thatpermits fluid communication between the first port and the third portand fluid communication between the second port and the fourth port (asindicated by the broken line of FIG. 1).

The refrigerant circuit (10) further includes a bypass line (40). Oneend of the bypass line (40) is connected to between the inflow side ofthe expander (23) and the second four-way switching valve (14), and theother end thereof is connected to between the outflow side of theexpander (23) and the second four-way switching valve (14). In otherwords, the bypass line (40) constitutes a bypass passage whichestablishes fluid communication between the entrance side and the exitside of the expander (23).

The bypass line (40) is provided with a bypass valve (41) which is acontrol valve. The bypass valve (41) is formed by a so-called electronicexpansion valve, wherein the valve opening of the bypass valve (41) isvariable by rotating its needle with a pulse motor or the like. When thevalve opening of the bypass valve (41) is changed, the flow rate ofrefrigerant flowing through the bypass line (40) varies. In addition,when the bypass valve (41) is placed in the fully closed position, thebypass line (40) enters the blocked state. As a result, every portion ofhigh-pressure refrigerant is delivered into the expander (23).

The controller (50) is configured, such that it regulates the capacityof the second compressor (22) and the flow rate of refrigerant in thebypass line (40) in order that the high pressure of the refrigerationcycle may assume a predetermined target value. More specifically, thecontroller (50) performs an operation of regulating the frequency of ACpower that is supplied to the second electric motor (32) and anoperation of regulating the valve opening of the bypass valve (41). Inaddition, the controller (50) performs also an operation of controllingthe capacity of the first compressor (21) by regulating the frequency ofAC power that is supplied to the first electric motor (31).

Operation Modes

With reference to FIGS. 1 and 2, space cooling and heating operations bythe air conditioner of the present embodiment are described. Point A,Point B, Point C, and Point D used in the description correspond,respectively, to Point A, Point B, Point C, and Point D shown in aMollier chart of FIG. 2. In addition, operations when the secondcompressor (22) is stopped and the bypass valve (41) is fully closed aredescribed here. These operations in such a state are performed in anoperation condition in which the ratio of the specific volume ofrefrigerant at the exit of an evaporator and the specific volume ofrefrigerant at the exit of a radiator agrees with the ratio of thedisplacement volume of the first compressor (21) and the displacementvolume of the expander (23).

Cooling Mode of Operation

During the cooling mode of operation, the first four-way switching valve(13) and the second four-way switching valve (14) each switch into thestate (indicated by the solid line of FIG. 1). If, in this state, thefirst electric motor (31) is energized, this causes refrigerant tocirculate through the refrigerant circuit (10), whereby a refrigerationcycle is carried out. At this time, the outdoor heat exchanger (12)operates as a radiator while, on the other hand, the indoor heatexchanger (11) operates as an evaporator. P_(H) (the high pressure ofthe refrigeration cycle) is set higher than P_(C) (the critical pressureof carbon dioxide as a refrigerant) (see FIG. 2).

High-pressure refrigerant in a state of Point A is expelled out of thefirst compressor (21). This high-pressure refrigerant flows into theoutdoor heat exchanger (12) by way of the first four-way switching valve(13). In the outdoor heat exchanger (12), the high-pressure refrigerantdissipates heat to outdoor air, is lowered in enthalpy without change inpressure (i.e., its pressure remains at a level of P_(H)), and changesstate into Point B.

High-pressure refrigerant exiting the outdoor heat exchanger (12) flowsinto the expander (23) by way of the second four-way switching valve(14). In the expander (23), the high-pressure refrigerant introducedthereinto expands and the internal energy of the high-pressurerefrigerant is converted into rotational power. As a result of expansionin the expander (23), the high-pressure refrigerant is lowered inpressure and enthalpy and changes state into Point C. That is, bypassage through the expander (23), the pressure of the refrigerant fallsfrom P_(H) down to P_(L).

Low-pressure refrigerant at a pressure level of P_(L) exiting theexpander (23) flows into the indoor heat exchanger (11) by way of thesecond four-way switching valve (14). In the indoor heat exchanger (11),the low-pressure refrigerant absorbs heat from indoor air, is increasedin enthalpy without change in pressure (i.e., its pressure remains at alevel of P_(L)), and changes state into Point D. In addition, in theindoor heat exchanger (11), indoor air is cooled by low-pressurerefrigerant, and the indoor air thus cooled is delivered back to theindoor space.

Low-pressure refrigerant exiting the indoor heat exchanger (11) is drawninto the first compressor (21) by way of the first four-way switchingvalve (13). The refrigerant drawn into the first compressor (21) iscompressed to a pressure level of P_(H), changes state into Point A, andis expelled from the first compressor (21).

Heating Mode of Operation

During the heating mode of operation, the first four-way switching valve(13) and the second four-way switching valve (14) each switch into thestate (indicated by the broken line of FIG. 1). If, in this state, thefirst electric motor (31) is energized, this causes refrigerant tocirculate through the refrigerant circuit (10), whereby a refrigerationcycle is carried out. At this time, the indoor heat exchanger (11)operates as a radiator while, on the other hand, the outdoor heatexchanger (12) operates as an evaporator. In addition, the high pressureof the refrigeration cycle (P_(H)) is set higher than the criticalpressure of carbon dioxide as a refrigerant (P_(C)), as in the coolingmode of operation (see FIG. 2).

High-pressure refrigerant in a state of Point A is expelled out of thefirst compressor (21). This high-pressure refrigerant flows into theindoor heat exchanger (11) by way of the first four-way switching valve(13). In the indoor heat exchanger (11), the high-pressure refrigerantdissipates heat to indoor air, is lowered in enthalpy without change inpressure (i.e., its pressure remains at a level of P_(H)), and changesstate into Point B. In addition, in the indoor heat exchanger (11),indoor air is heated by high-pressure refrigerant. The indoor air thusheated is delivered back to the indoor space.

High-pressure refrigerant exiting the indoor heat exchanger (11) flowsinto the expander (23) by way of the second four-way switching valve(14). In the expander (23), the high-pressure refrigerant introducedthereinto expands and the internal energy of the high-pressurerefrigerant is converted into rotational power. As a result of expansionin the expander (23), the high-pressure refrigerant is lowered inpressure and enthalpy and changes state into Point C. That is, bypassage through the expander (23), the pressure of the refrigerant fallsfrom P_(H) down to P_(L).

Low-pressure refrigerant at a pressure level of P_(L) exiting theexpander (23) flows into the outdoor heat exchanger (12) by way of thesecond four-way switching valve (14). In the outdoor heat exchanger(12), the low-pressure refrigerant absorbs heat from outdoor air, isincreased in enthalpy without change in pressure (i.e., its pressureremains at a level of P_(L)), and changes state into Point D.

Low-pressure refrigerant exiting the outdoor heat exchanger (12) isdrawn into the first compressor (21) by way of the first four-wayswitching valve (13). The refrigerant drawn into the first compressor(21) is compressed to a pressure level of P_(H), changes state intoPoint A, and is expelled from the first compressor (21).

Operation of Controller

The controller (50) regulates the capacity of the second compressor (22)and the flow rate of refrigerant in the bypass line (40) in order thatthe high pressure of the refrigeration cycle (P_(H)) may assume apredetermined target value.

The controller (50) is fed a measured value of the low pressure of therefrigeration cycle (P_(L)), and a measured value of the temperature ofrefrigerant (T) at the exit of the outdoor heat exchanger (12)functioning as a radiator or at the exit of the indoor heat exchanger(11) functioning as a radiator. In addition, the controller (50) is feda measured value of the high pressure of the refrigeration cycle(P_(H)). And, the controller (50) regulates the frequency of AC powerthat is supplied to the second electric motor (32) and the valve openingof the bypass valve (41) in order that the measured value of thehigh-pressure of the refrigeration cycle (P_(H)) may assume apredetermined target value.

Setting of Target Value

Based on input measured values, i.e., a measured value of thelow-pressure (P_(L)) and a measured value of the refrigerant temperature(T), the controller (50) sets, as a target value, an optimum value forthe high pressure of the refrigeration cycle. In doing so, thecontroller (50) computes, by making utilization of pre-storedcorrelation equations, tables of numerical data, or the like, an optimalvalue for the high pressure of the refrigeration cycle, i.e., ahigh-pressure value capable of maximizing the COP of the refrigerationcycle, and sets the result as a target value. Then, the controller (50)compares an input measured value of the high pressure (P_(H)) with theset target value and performs the following operations according to thecompare result.

When Measured Value of High Pressure P_(H)=Target Value

When a measured value of the high pressure (P_(H)) agrees with thetarget value, neither the capacity of the second compressor (22) nor theflow rate of refrigerant in the bypass line (40) has to be changed.Therefore, the controller (50) controls the frequency of AC power thatis supplied to the second electric motor (32) and the valve opening ofthe bypass valve (41), such that they remain unchanged. In other words,if the second compressor (22) is being at rest, then the secondcompressor (22) will be held in the stopped state. In addition, if thebypass valve (41) is being fully closed, then the bypass valve (41) willbe held in the fully closed state.

When Measured Value of High Pressure P_(H)>Target Value

If, in a certain operation state, both the first compressor (21) and thesecond compressor (22) are being operated when a measured value of thehigh pressure (P_(H)) is greater than the target value, it may bedecided that the sum total of the displacement volume of the firstcompressor (21) and the displacement volume of the second compressor(22) is excessive. Based on such a decision, the controller (50) reducesthe frequency of AC power that is supplied to the second electric motor(32) and lowers the rotational speed of the second compressor (22),thereby to reduce the displacement volume of the second compressor (22).That is, the controller (50) reduces the capacity of the secondcompressor (22).

If, even when the second compressor (22) is brought into a stop, ameasured value of the high pressure (P_(H)) is still greater than thetarget value, it may be decided that the displacement volume of theexpander (23) is excessively small. To deal with this, the controller(50) places the bypass valve (41) in the open state for introducingrefrigerant into both of the expander (23) and the bypass line (40).That is, refrigerant flows through not only the expander (23) but alsothe bypass line (40), thereby assuring the circulation amount ofrefrigerant.

When Measured Value of High Pressure P_(H)<Target Value

If, in a certain operation state, the second compressor (22) is at restwhile the bypass valve (41) is in the open state when a measured valueof the high pressure (P_(H)) falls below the target value, it may bedecided that the sum total of the flow rate of refrigerant in theexpander (23) and the flow rate of refrigerant in the bypass line (40)is excessively great. To deal with this, the controller (50) reduces thevalve opening of the bypass valve (41) for decreasing the flow rate ofrefrigerant in the bypass line (40).

If, even when the bypass valve (41) is brought into a fully closedposition, a measured value of the high pressure (P_(H)) still fallsbelow the target value, it may be decided that the displacement volumeof the first compressor (21) is excessively small. Therefore, in thiscase, the controller (50) starts supplying power to the second electricmotor (32) for activating the second compressor (22). Thereafter, thecontroller (50) increases or decreases the frequency of AC power that issupplied to the second electric motor (32) according to need, wherebythe rotational speed of the second compressor (22) is varied. In thisway, the displacement volume of the second compressor (22) is regulated.To sum up, the controller (50) controls the capacity of the secondcompressor (22).

If, even when the rotational speed of the second compressor (22) isincreased to a maximum (i.e., even when the capacity of the secondcompressor (22) is increased to a maximum), a measured value of the highpressure (P_(H)) still falls below the target value, it may be decidedthat the displacement volume of the expander (23) is excessively great.Therefore, in this case, the controller (50) reduces the frequency of ACpower that is supplied to the first electric motor (31), whereby therotational speed of the expander (23) is lowered. In this way, thedisplacement volume of the expander (23) is cut down.

Effects of Embodiment 1

In the air conditioner of the first embodiment, in the refrigerantcircuit (10) the second compressor (22), not connected to the expander(23), is arranged in parallel with the first compressor (21). Because ofthis arrangement, even in such an operation condition that the volume ofdisplacement only by the first compressor (21) connected to the expander(23) becomes deficient, it is possible to compensate such a deficiencyby setting the second compressor (22) in operation, and therefrigeration cycle is continued in an adequate operation condition.

Here, suppose the temperature of outside air decreases in an operationcondition in which a measured value of the high pressure (P_(H)) agreeswith the target value when the second compressor (22) is stopped and thebypass valve (41) is closed in the air conditioner. At this time,refrigerant at the exit of the outdoor heat exchanger (12) (operating asa radiator) changes state from Point B to Point B′ as shown in FIG. 3A,if the air conditioner is in a space cooling mode of operation. In otherwords, the temperature of refrigerant at the exit of the outdoor heatexchanger (12) decreases and, as a result, the specific volume ofrefrigerant diminishes. On the other hand, if the air conditioner is ina space heating mode of operation, the pressure of refrigerant in theoutdoor heat exchanger (12) (operating as an evaporator) is lowered fromP_(L) down to P_(L)′, as shown in FIG. 3B. That is, the low pressure ofthe refrigeration cycle is lowered and, as a result, the specific volumeof refrigerant at the outdoor heat exchanger's (12) exit increases.

When the temperature of outside air decreases as described above, it isrequired for a conventional air conditioner without the secondcompressor (22) to establish a balance in displacement volume betweenthe compressor side and the expander side by introducing refrigerant,the specific volume of which is pre-increased by expansion in anexpansion valve positioned upstream of the expander (23), into theexpander (23).

On the other hand, in the present embodiment, the displacement volume ofthe compressor side is balanced with the displacement volume of theexpander side by operating both of the first compressor (21) and thesecond compressor (22). Because of this, if the air conditioner is in aspace cooling mode of operation, a refrigeration cycle as indicated bythe solid line of FIG. 3A becomes possible to perform by intactlyintroducing refrigerant in the state of Point B′ into the expander (23),as shown in FIG. 3A. On the other hand, if the air conditioner is in aspace heating mode of operation, a refrigeration cycle as indicated bythe solid line of FIG. 3B becomes possible to perform by intactlyintroducing refrigerant in the state of Point B into the expander (23),as shown in FIG. 3B.

To sum up, even in an operation condition in which refrigerant has to beflowed into the expander (23) after being pre-expanded by an expansionvalve or the like as conventionally required, it is possible tointroduce high-pressure refrigerant after heat dissipation into theexpander (23) without the necessity for pre-expansion. As a result, thedegradation of power produced in the expander (23) is avoided.Accordingly, in accordance with the present embodiment, stablerefrigeration cycle operations are possible to perform, regardless ofthe operation condition, thereby making it possible to improve the COPof the air conditioner.

On the other hand, suppose the temperature of outside air increases inan operation condition in which a measured value of the high pressure(P_(H)) agrees with the target value when the second compressor (22) isstopped and the bypass valve (41) is closed in the air conditioner. Atthis time, refrigerant at the exit of the outdoor heat exchanger (12)(operating as a radiator) changes state from Point B to Point B′ asshown in FIG. 4A, if the air conditioner is in a space cooling mode ofoperation. In other words, the temperature of refrigerant at the exit ofthe outdoor heat exchanger (12) increases and, as a result, the specificvolume of refrigerant increases. On the other hand, if the airconditioner is in a space heating mode of operation, the pressure ofrefrigerant in the outdoor heat exchanger (12) (operating as anevaporator) increases from P_(L) up to P_(L)′, as shown in FIG. 4B. Thatis, the low pressure of the refrigeration cycle increases and, as aresult, the specific volume of refrigerant at the outdoor heatexchanger's (12) exit diminishes.

When the temperature of outside air increases as described above, in thepresent embodiment the bypass valve (41) is placed in the open state soas to introduce refrigerant also into the bypass line (40) forestablishing a balance in volume flow rate between the compression sideand the expansion side. And, if the air conditioner is in a spacecooling mode of operation, refrigerant in the state of Point C′ past theexpander (23) and refrigerant in the state of Point E past the bypassvalve (41) flow into the indoor heat exchanger (11) operating as anevaporator, as shown in FIG. 4A. In addition, if the air conditioner isin a space heating mode of operation, refrigerant in the state of PointC′ past the expander (23) and refrigerant in the state of Point E pastthe bypass valve (41) flow into the outdoor heat exchanger (12)operating as an evaporator, as shown in FIG. 4B.

Accordingly, in accordance with the present embodiment, even in anoperation condition in which the displacement volume of the expander(23) alone is not sufficient enough to secure a required circulationamount of refrigerant, a deficiency in the refrigerant flow rate iscovered by introduction of high-pressure refrigerant into the bypassline (40), thereby making it possible to assure continuation of therefrigeration cycle in an adequate operation condition.

It is true that, if a portion of high-pressure refrigerant is introducedinto the bypass line (40), the amount of high-pressure refrigerantflowing into the expander (23) is reduced by an amount correspondingthereto, therefore causing the degradation of power produced in theexpander (23). However, when designing air conditioners, compressors andexpanders (23) are generally designed so as to achieve a maximum COP inoperation conditions of most frequency, and the frequency of operationconditions that require the introduction of refrigerant into the bypassline (40) is not very high. And, when trying to deal with such anoperation condition of low frequency by controlling the capacity of thesecond compressor (22), this rather causes the COP of the airconditioner to fall in operation conditions of high frequency becauseof, for example, the existence of the loss of power in the electricmotors (31, 32).

Accordingly, in accordance with the present embodiment, refrigerationcycles are continued by introducing refrigerant into the bypass line(40) in special operation conditions of low frequency and the usabilityof the air conditioner is maintained at high level while, on the otherhand, high COPs are achieved by introducing high-pressure refrigerantinto the expander (23) in normal operation conditions of high frequency.

Embodiment 2 of Invention

A second embodiment of the present invention is an embodiment in whichthe refrigerant circuit (10) and the controller (50) of the firstembodiment are modified in configuration. Hereinafter, differencesbetween the present embodiment and the first embodiment will bedescribed.

As shown in FIG. 5, in the refrigerant circuit (10) of the presentembodiment, the bypass line (40) and the bypass valve (41) are omitted.Accordingly, the controller (50) of the present embodiment is configuredso as to regulate only the capacity of the first and second compressors(21, 22). In other words, if a measured value of the high pressure(P_(H)) exceeds the target value, the controller (50) reduces therotational speed of the second electric motor (32), thereby to decreasethe capacity of the second compressor (22). On the other hand, if ameasured value of the high pressure (P_(H)) falls below the targetvalue, the controller (50) increases the rotational speed of the secondelectric motor (32), thereby to increase the capacity of the secondcompressor (22).

For example, in the case where the range of operation conditions thatthe air conditioner should deal with is not very wide, and in the casewhere the second compressor (22) is extensively regulatable in capacitywhile the second compressor (22) maintains high efficiency, the bypassline (40) and the bypass valve (41) may be omitted.

INDUSTRIAL APPLICABILITY

As has been described above, the present invention is useful forrefrigeration apparatuses provided with expanders.

1. A refrigeration apparatus which performs a refrigeration cycle bycirculating refrigerant through a refrigerant circuit (10), comprising:an expander (23), disposed in said refrigerant circuit (10), forproducing power by expansion of high-pressure refrigerant; a firstcompressor (21), disposed in said refrigerant circuit (10) and connectedto a first electric motor (31) and said expander (23), for compressingrefrigerant when driven by power produced in said first electric motor(31) and said expander (23); and, a variable capacity second compressor(22), disposed in parallel with said first compressor (21) in saidrefrigerant circuit (10) and connected to a second electric motor (32),for compressing refrigerant when driven by power produced in said secondelectric motor (32).
 2. The refrigeration apparatus of claim 1, furthercomprising: control means (50) for regulating the capacity of saidsecond compressor (22) so that the high pressure of said refrigerationcycle assumes a predetermined target value.
 3. The refrigerationapparatus of claim 1, further comprising: a bypass passage (40) forestablishing fluid communication between an entrance and exit sides ofsaid expander (23) in said refrigerant circuit (10); and, a controlvalve (41) for regulating the flow rate of refrigerant in said bypasspassage (40).
 4. The refrigeration apparatus of claim 3, furthercomprising: control means (50) for regulating the capacity of saidsecond compressor (22) and the valve opening of said control valve (41)so that the high pressure of said refrigeration cycle assumes apredetermined target value.
 5. The refrigeration apparatus of claim 4,wherein said refrigeration apparatus is configured so that: when saidcontrol valve (41) is in the fully closed state and the high pressure ofsaid refrigeration cycle falls below said predetermined target value,said control means (50) sets said second compressor (22) in operationand regulates the capacity of said second compressor (22); and, whensaid second compressor (22) is in the stopped state and the highpressure of said refrigeration cycle exceeds said predetermined targetvalue, said control means (50) places said control valve (41) in theopen state and regulates the valve opening of said control valve (41).6. The refrigeration apparatus of claim 1, wherein: said refrigerantcircuit (10) is filled up with carbon dioxide as a refrigerant, and thehigh pressure of said refrigeration cycle performed by circulatingrefrigerant through said refrigerant circuit (10) is set higher than thecritical pressure of carbon dioxide.